PAUSE. Rewind. Suddenly the outward rush of 200 billion galaxies slips into reverse. Instead of expanding at pace, the universe is now imploding like a deflating balloon: faster and faster, smaller and smaller, everything hurtling together until the entire cosmos is squeezed into an inconceivably hot, dense pinprick. Then pshhht! The screen goes dead.
According to the big bang theory – our best explanation for why space is expanding – everything exploded from nothing about 13.8 billion years ago. Cosmologists have been able to wind things back to within a tiny fraction of a second of this moment. But now they’re stuck.
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The trouble is, our understanding of space-time, and gravity in particular, is built from Einstein’s equations of general relativity, whereas the extreme conditions of the very early universe can only be described by quantum mechanics. No one knows how to reconcile the two to take us further back. “The rules we have simply don’t work in that regime,” says at Imperial College London. “Nothing makes sense any more.”
That’s a problem for our origin story. Did time begin with the big bang? Or was there an epoch before it?
Some insist that if we rewind the universe far enough, time just stops.But of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, is having none of it.
“It’s a cute idea but there’s not much evidence for it,” he says. In fact, Smolin wants to see the idea that the universe has a starting point dropped entirely. We can only hope to explain why our universe is the way it is, he says, if there was something before the big bang. It’s about cause and effect; to arrive at satisfying explanations for why things are as they are, we draw on previous events that led to the conditions we see.
One possibility is that instead of a bang, there was a bounce. In this scenario, rewinding our universe would not take us back to a point containing infinite mass in an infinitely small space. Instead it would take us through the unimaginably hot, dense beginning of our universe and out the other side into the unimaginably hot, dense ending of a previous universe.
Acknowledging that something came before the big bang would open the door to explanations of how the universe came to be just right for life. Otherwise we either have to accept that our perfectly crafted universe was just a fluke or that it is one of an infinite number of universes, most of which look very different, and we only think of it as special because we’re in it.
Or perhaps our bang was one of many. Some think our universe popped into existence as a bubble in a frothing sea of universes. This idea gives our universe a beginning but introduces an everlasting multiverse.
Can we ever really know? Smolin thinks we could soon see clues in the cosmic microwave background – radiation given off by the early universe. It’s possible that ripples in space-time known as gravitational waves might also survive a bounce.
Contaldi is not holding his breath. We need a working theory of quantum gravity and a new understanding of time, he says. Without that, “I don’t think we have the tools to even pose the question properly.”
Read more: “10 mysteries that physics can’t answer… yet”
This article appeared in print under the headline “What came before the big bang?”
